This invention pertains to a silicon nitride (Si.sub.3 N.sub.4) ceramic body and a process for preparing the ceramic body.
Silicon nitride ceramics are recognized for their excellent mechanical and physical properties, including good wear resistance, low coefficient of thermal expansion, good thermal shock resistance, high creep resistance and high electrical resistivity. In addition, silicon nitride ceramics are resistant to chemical attack, particularly to oxidation. Because of these attributes, silicon nitride is useful in a variety of wear and high temperature applications, such as cutting tools and parts in pumps and engines.
Failure of silicon nitride ceramics is generally associated with brittleness and flaws. The object therefore is to prepare a silicon nitride ceramic with high fracture toughness (KIC) and strength. Fracture strength is directly proportional to the fracture toughness and inversely proportional to the square root of the flaw size. High fracture toughness combined with small flaw size is therefore highly desirable. Monolithic silicon nitride, however, has a relatively low fracture toughness of about 5 MPa (m).sup.1/2.
U.S. Pat. No. 4,543,345 teaches that the addition of silicon carbide whiskers to ceramic materials can result in an increase in the fracture toughness. Silicon carbide whiskers described in this patent have a single crystal structure and are in a size range of about 0.6 .mu.m in diameter and about 10 .mu.m to about 80 .mu.m in length. This technique, however, does not provide significant toughening in the case of silicon nitride ceramics. Moreover, the use of silicon carbide whiskers is associated with serious processing problems. The whiskers have a tendency to agglomerate and settle. It is difficult to deagglomerate the whiskers without significantly destroying the whiskers' length. In addition, the whiskers are difficult to manufacture; thus, they display inconsistent properties and are costly. It would be highly desirable to have a silicon nitride ceramic of high fracture toughness which does not require the presence of silicon carbide whiskers.
U.S. Pat. No. 4,669,890 discloses a highly dense silicon nitride sintered body containing predetermined amounts of yttria (Y.sub.2 O.sub.3), magnesia (MgO), and ceria (CeO) and a method for preparing such a body. The amounts are, based upon weight of the sintered body, 2-15% by weight of Y as Y.sub.2 O.sub.3, 1-10% by weight of Ce as CeO, 1-10% by weight of Mg as MgO and 75-95% by weight of Si.sub.3 N.sub.4. The bodies have, as shown in Table I, a demonstrated room temperature mechanical strength (four point bending test) of 670-750 MPa. It would be very desirable to have a silicon nitride ceramic with a fracture strength (room temperature) of 825 MPa or higher and a fracture toughness greater than about 6 MPa (m).sup.1/2.
U.S. Pat. No. 4,025,351 and related U.S. Pat. No. 4,004,937 disclose a silicon nitride-based ceramic formed from a powder mixture containing 60-92 mole percent silicon nitride with the balance being metal oxide. The metal oxide component consists of at least one of MgO, ZnO and NiO and at least one of Al.sub.2 O.sub.3, Cr.sub.2 O.sub.3, Y.sub.2 O.sub.3, TiO.sub.2 and SnO.sub.2. The mole ratio of the first group to the second group ranges from 1:9 to 9:1. In one embodiment, the mixed metals are heated to between 1600 and 1800.degree. C. for 2 to 3 hours. In a second embodiment, the mixed metals are first converted to a spinel which is then ground, mixed with the silicon nitride and sintered. A small quantity of CaO or CoO may also be included in the silicon nitride-based ceramic.
U.S. Pat. No. 4,406,668 discloses a coated ceramic cutting tool comprising a densified silicon nitride substrate body having at least one hard, adherent coating layer consisting essentially of a refractory metal nitride. The substrate body consists essentially of a first phase of silicon nitride and a second refractory phase comprising silicon nitride and an effective amount of a single densification additive. The densification additive is selected from the group consisting of silicon dioxide, aluminum oxide, magnesium oxide, yttrium oxide, zirconium oxide, hafnium oxide, the lanthanide rare earth oxides, and mixtures thereof. The coating is a nitride of titanium, vanadium, chromium, zirconium, niobium, molybdenum, hafnium, tantalum, tungsten or mixtures thereof.
It is known that the high temperature strength of hot-pressed silicon nitride ceramics can be increased by crystallization of the grain-boundary glass phase (second phase). This has been demonstrated in a hot-pressed composite containing beta(.beta.)-silicon nitride and a crystalline second phase of Si.sub.3 N.sub.4 .multidot.Y.sub.2 O.sub.3, as reported by Akihiko Tsuge et al. in the Journal of the American Ceramics Society, 58, 323-326 (1975). However, the fracture toughness of this silicon nitride is only 5-6 MPa (m).sup.1/2.
It is also known that the presence of single crystal .beta.-silicon nitride whiskers with a high aspect ratio can increase the fracture toughness of silicon nitride ceramics, as reported by F. F. Lange, in the Journal of the American Ceramics Society, 62 (12), 1369-1374, (1983). "Aspect ratio" is defined as the ratio of length to diameter or width. Thus, single crystal whiskers with a high aspect ratio are fibrous in nature. If such whiskers are also strong, crack propagation must take a tortuous path around the whiskers, thereby leading to high fracture toughness. The transformation of alpha(.alpha.)-silicon nitride to .beta.-silicon nitride takes place above 1600.degree. C.; however, crystals of the beta phase precipitate usually as a mixture of equiaxed grains and elongated grains with a low aspect ratio. Reproducible control of the aspect ratio is a difficult problem.
Typically, the prior art is silent with regard to aspect ratio and fracture toughness of silicon nitride ceramics. U.S. Pat. No. 4,279,657, for example, discloses a powder dispersion containing silicon nitride and magnesium oxide which is hot-pressed to form a light-transmitting silicon nitride ceramic. The ceramic is disclosed to comprise more than 50 weight percent .beta.-silicon nitride ranging in grain size from 1 .mu.m to about 10 .mu.m, but typically less than 5 .mu.m. This patent teaches that impurities, such as calcium, in a total amount greater than about 0.1 weight percent are undesirable. U.S. Pat. No. 4,227,842 discloses a cutting tool consisting essentially of beta phase silicon nitride and yttrium oxide. The tool is prepared by hot-pressing a powder mixture of c-silicon nitride and yttrium oxide to obtain a ceramic of 100 percent theoretical density. U.S. Pat. No. 4,652,276 teaches a cutting tool comprising a granular phase consisting essentially of .beta.-silicon nitride and an intergranular amorphous phase consisting essentially of magnesium oxide from about 0.5 to about 10 weight percent, yttrium oxide from about 2.5 to about 10 weight percent, silicon oxide in an amount-less than about 2.5 weight percent, and the balance less than 5 weight percent impurities such as aluminum. The tool is prepared by hot-pressing.
It would be very desirable to have a silicon nitride ceramic of high fracture toughness and high fracture strength. It would be advantageous if such a strong silicon nitride ceramic could be prepared without silicon carbide reinforcing whiskers. Moreover, it would be highly desirable to have a process which would be reproducible, inexpensive, and amenable to industrial scale-up for preparing such a tough and strong silicon nitride ceramic.